Narrow gap welding is a process wherein successive weld passes are applied directly on top of one another in a narrow groove or gap. Narrow gap welding techniques are particularly applicable to those situations where thick metal elements are to be joined by applying weld material in a narrow groove formed by opposing wall surfaces of the pieces to be joined. Inadequate sidewall fusion is a major problem in achieving satisfactory narrow gap welds. Because of this fusion problem, other techniques such as electroslag welding are commonly employed at the present time.
Narrow gap welding techniques are primarily useful for applications including but not limited to nuclear reactor pressure vessels, deep diving submarine hulls and heavy piping. All of these applications require high-quality welds that are impact resistant. Unfortunately, electroslag welds are impact deficient.
Several different types of narrow gap welding devices are presently known. Some utilize various oscillation or weld wire position variation devices which may employ weld head or weld wire drive systems that tend to require frequent service as a result of the harsh welding environment. U.S. Pat. No. 4,091,258 to Kano et al is typical of the type of torch which uses a linear mechanical oscillation to achieve the necessary sidewall fusion to affect an acceptable weld. U.S. Pat. No. 4,095,085 to Tomita et al discloses a torch which utilizes linear mechanical oscillation with electro-magnetic arc deflection for achieving adequate sidewall fusion. The arc is deflected by oscillating the welding wire or electrode.
Other narrow gap welding processes are described in U.S. Pat. No. 3,679,866 to Arikawa et al and in U.S. Pat. No. 3,328,556 which surveys several different narrow gap welding processes and apparatus.
Most of these prior art torches can potentially produce defective welds as a result of operator error or because the complex system fails to reverse the contact tip attitude as required. No electro-mechanical system for centering the welding electrode in the gap is presently known which eleminates overall system complexity and maintenance problems.
Other devices which utilize probe-type centering devices are subject to providing spurious error correction signals because of weld spatter which interferes with the probe contacts.
Twisted wire consumable electrodes provide rapid alternating arcing from sidewall to sidewall in a narrow gap which ensures adequate fusion and which, moreover, can accommodate variances of plus or minus 1/8 of an inch from the center of the groove without serious danger resulting from lack of fusion. Narrow gap gas arc welding devices employing twisted wire center electrodes are known, see for example, Table 1-1 of the publication entitled "Current Status of Practical Application of Narrow Gap Welding in Japan," September, 1981 by the Japanese Pressure Vessel Research Counsel and also FIG. 9 of "Twist Arc Welding Processes for Narrow Gap Welding," March, 1982 by Kobe Steel, Ltd., (hereinafter "Twist Arc Welding").
Up to the present time, none of the proposed arrangements for narrow gap welding have proven to be commercially successful. The reasons for lack of success have been manifold, but usually relate to the difficulty in obtaining high-quality welds at an economical rate. For instance, the torch disclosed in the Twist Arc Welding publication, while representing a simplification over the electrode oscillating devices discussed above, is nevertheless inefficient in that it requires considerable amounts of shield gas flow for stable welding operation - in fact, in excess of 125 ft/hr. of expensive inert gas. This appears to be due to the fact that the shielding gas is used not only to shield the weld but also to cool the gas bars of the torch. Such high gas flow rates are not only expensive to maintain but also results in accelerated gas errosion of the torch bars and associated upstream gas handling components.